Capacitors are used for a variety of purposes including energy storage, signal coupling, motor starting, power factor correction, voltage regulation, tuning, resonance and filtration. In series and shunt implementations, there are many operational advantages, both transient and steady state, for implementing capacitors in general AC networks.
Network efficiency is increased with power factor improvement, during transient conditions. Transient applications of series capacitors include voltage surge protection, motor starting, current limiting, switching operations and the like. Series capacitors can moderate the effects of AC network faults and other transient conditions. For example, low power factor transient currents are associated with magnetic inrush currents due to motor starting, transformer inrush and fault currents. Series capacitance improves the overall power factor and network voltage regulation during these transient conditions. Series capacitor banks also demonstrate a degree of current limiting due to the series impedance of the capacitor. This reduces fault currents and thus reduces generator, transformer, switchgear, bus and transmission line size requirements. The capacitor in series with the fault acts as a current limiting device. Tuned circuits composed of inductors and capacitors (LC circuits) are used for filtration. A high induction series version can dramatically increase network fault impedance by deliberately shorting out the capacitor bank. A series capacitor bank is typically coupled to a transformer. Transformer opposition to instantaneous current change combines with capacitor opposition to instantaneous voltage change. These characteristics lead to greater instantaneous network voltage stability as a result of increased use of series capacitor banks. Secondary effects include voltage surge protection, demand factor improvement and voltage regulation. Instantaneous power transfer efficiency can be improved with proper capacitor use. While these many series capacitor advantages are well known, and proven in the lab, unit cost and size requirements have prevented their general implementation.
AC network steady state characteristics are also improved through the incorporation of capacitors. High capacitance, series applications impress a low steady state AC voltage on the capacitor. This is helpful when electrical transfer devices are used in conjunction with series capacitor banks. Electrical wave distortion is similarly reduced with increasing capacitance. Steady state series capacitor applications include motor running, filtration, power factor correction, efficient power transfer, voltage boosting and the like. Series capacitor banks allow induction generators to power induction motors, by providing the required magnetizing [VARs] for both devices. This can also improve the power quality, while reducing the cost of electric grid alternative sources, emergency power supplies, mobile equipment and portable generators. Mechanical stress associated with bringing additional generation capacity, on line, to synchronous operation, can be moderated by the presence, of series capacitive coupling.
The two major capacitor categories are polar and non-polar. There are many realizations of each category. Due to their unidirectional, forward biasing requirements, polarized capacitors are mostly used in DC and small AC signal applications. Polarized capacitors are widely used in DC filtering applications, such as output stages of DC power supplies. Audible frequency (music) amplifiers use a DC biased polarized capacitor to couple signals. Conversely, non-polarized capacitors are generally useful in both DC and general AC applications. Unfortunately, non-polarized capacitors—especially in series applications—are not well suited for many AC and DC uses due to their limitations in size, capacitance, weight, efficiency, energy density and cost. The use of undersized non-polar capacitor banks causes significant current waveform distortion and a large voltage drop across the capacitor, which results in energy losses and poor AC voltage regulation at the AC load.
Conversely, polarized capacitors, as well as other polarized electric charge storage (PECS) devices, have a low cost per unit of capacitance, as well as smaller mass and dimensions, as compared with nonpolar capacitors. These characteristics favor their use over non-polarized capacitors. They also exhibit a relatively low series AC resistance at power frequencies. However, they may only be effectively operated with positive “forward” voltages relative to their positive and negative poles. A reversed voltage of any significant magnitude causes the capacitor to short, which usually results in an explosion that can be comparable to that of a hand grenade. In fact, with solid tantalum capacitors, this short circuit failure mode includes spontaneous combustion. Thus, polarized capacitors, for the most part, have not been amenable for general AC applications.
FIG. 1 models the normal operation of a polarized aluminum electrolytic capacitor as well as circuit operation in over-voltage and reverse bias voltage mode. The model consists of series inductor 101, series resistor 102, parallel resistor 103, zener diode 104 and polarized capacitor 105. Zener diode 104 models the forward and reverse shorting condition present when the impressed voltage exceeds a reverse bias voltage of 1.5 Volts or a forward bias condition of approximately 50 Volts over the rated working DC voltage (WVDC) of the capacitor. Inductor 101 is suitable for modeling the self-resonant frequency of the capacitor. The series resistor 102 models the (small, mΩ) equivalent series resistance (ESR) measured in capacitor operation. The parallel resistor 103 models the (large, MΩ) equivalent parallel resistance measured in capacitor DC leakage current phenomenon. In low frequency operation, forward biased voltage within the device working voltage conditions will allow signal current flow through the directional capacitor 105. Reverse bias conditions will occasion a short through diode 104.
The capacitor will suitably operate continuously between zero volts and the rated working DC voltage. A reverse bias voltage of up to about 1.5 volts DC to a rated forward bias surge voltage defines the outer limits of appropriate transient use of the capacitor. Capacitor operation outside this wider voltage envelope will cause short circuit conditions. There is typically a third, higher impulse voltage parameter. Excessive forward voltage on the capacitor will cause a reverse current flow through zener diode 104. This electrical behavior is schematically modeled by depicting a zener diode 104 in parallel, but with opposite polarity alignment than the polar capacitor. Shorting through diode 104, in either direction permits excessive current, heat buildup, which eventuates capacitor failure. This is why a single polarized capacitor fails in normal AC operation.
FIG. 2 depicts a simple circuit realization 250 that illustrates a typical prior art use of a DC biased polarized capacitor in a small AC signal coupling application. This circuitry is commonly used as a laboratory exercise for undergraduate analog electronic students and is employed in multi-stage amplifiers. Circuit 250 includes an AC signal source 255 superimposed upon a DC bias voltage source 260, a capability of lab power supplies. The AC signal is coupled to the load 266, while the DC bias voltage is blocked by and positively biases a polarized capacitor 262. The capacitor and DC bias voltage are selected so that the superimposed AC and DC voltages are at all times within the proper voltage window. The AC source output section conducts the entire DC power source output, and vices versa. As the AC signal increases in magnitude relative to the capacitor rated DC working voltage, waveform distortion in the form of clipping occurs. Thus, the lowest waveform distortion occurs for small AC signals. The magnitude of the bias voltage is typically on the order of half the rated capacitor DC working voltage. The fidelity of AC waveform transfer improves as the magnitudes of the AC voltage signal, and the AC current are decreased.
A non-polarized capacitor 264 is shown in parallel with the polarized capacitor 262 for “polishing.” Non-polarized polishing capacitors may be used for fine-tuning resonance, adjusting capacitance to current ratio, reducing ESR, adjusting bandwidth, improving waveform transfer, flattening the frequency response and improving other such application specific aspects. The capacitance of the polarized capacitor 262 typically may exceed that of the polishing capacitor 264 by approximately two orders of magnitude. The non-polarized polishing capacitor works to reduce distortion of the signal.
FIG. 3 shows Circuit 300, which includes AC source 305, anti-series polarized capacitors 312, 314, collectively referred to as 310 and AC load 320. The polarity marks, above the caps, show an instantaneous forward bias condition of capacitor 312 and a simultaneous reverse bias condition of capacitor 314, which occurs during a positive phase of AC source 305. (Of course, the polarities would be reversed during the negative phase.)
Anti-series configurations of polarized capacitors will operate transiently, or in current limited applications. Such a conventionally implemented, anti-series configuration exploits the previously described internal zener diode-like behavior. It is typically used in single-phase motor starting applications and is plagued by overheating and short life due to reverse bias shorting. When capacitor 312 is forward biased by the AC source, capacitor 314 is reverse biased and shorts the half wave current to the load 320. On the next half wave, capacitor 314 is forward biased while capacitor 312 is shorted. This conventional anti-series configuration is notable for a DC bias condition, which oscillates on a sub-cycle (half cycle) basis.
With reference to FIG. 4, U.S. Pat. Nos. 4,672,289 and 4,672,290 to Ghosh teach an improved scheme for implementing anti-series, polarized capacitors in AC environments. Circuit 460 is shown in FIG. 4. Circuit 460 includes polarized capacitors 462, 464 and diodes 466, 468 in series with AC source 461 for driving AC load 470. Anti-series symmetrical polarized capacitors 462, 464 are in parallel with oppositely aligned anti-series diodes 466, 468. In operation, a parallel “shunt” diode (466, 468), clamps the maximum instantaneous negative voltage across each capacitor, which protects each polarized capacitor from being excessively reverse biased. The Ghosh circuit provides external discrete diodes to shunt the reverse currents away from each capacitor. The internal zener diode-like behavior is reduced. This reduces the heat build-up in the capacitors and extends their expected life.
Unfortunately, however, this shunting diode solution has certain material drawbacks. Each capacitor polarity is subjected to the full AC voltage, across the assembly, for one half of the AC waveform. Thus for a short circuit, motor starting, transformer inrush or similar condition, the entire AC source tension is impressed across the terminals, of each anti-series capacitor, and diode assembly, with a 50% duty cycle. No volt divider is present. Thus, the realizable AC ripple voltage is limited to available diode voltage ratings, for a given level of AC signal distortion. In addition, each polarized capacitor is subjected to a low voltage, reverse bias condition approximately 50% of the time. The diodes distort the AC network voltage waveform. Moreover, the self-biasing circuitry is not amenable to diode current limitation. These are problems in the steady state condition, due to heat loss, current waveform distortion and diode size requirements. These are even more significant problems for semiconductors in transient, fault, magnetizing inrush, resonance and/or starting applications. The entire circuit current passes through each diode with a 50% duty cycle in both the steady state and the transient case. This results in a significant heat loss through the diodes. Also, the self-bias DC voltage oscillation perturbs the system ground reference and further adds to the heat dissipation. AC signal distortion is present due to clipping as a result of inadequate DC bias voltage relative to the AC signal size. The energy required for capacitor charge reformation per half cycle is a further energy loss. In addition, this prior art solution is not suitable for use with other polarized charge storage devices such as many electrochemical batteries.
Furthermore, the circuit exhibits an absence of economy of scale for increased current requirements. If the capacitor bank amperage rating is doubled, so too must the diodes, heat sinks and the like. This constitutes a major capital expense in high current AC applications. If additional series diodes are required to increase the realizable voltage level, the additional diodes must have the same ampacity as the existing diodes. The forward voltage drop of, each existing diode, is matched, by the forward voltage drop of, each additional unit. Thus, power loss and heat generation increase proportionally. Also, the dead-zone about zero, of each diode, is multiplied, by the number of diodes in series.
This waveform distortion, due to the anti-series diode placement, e.g., in the Ghosh circuit, and the internal zener diode behavior in the conventional anti-series arrangement is thus intractable. In addition, the Ghosh and conventional circuits have an ongoing oscillatory effect on the system DC ground reference. These problems make the conventional and Ghosh devices unsuitable for general AC applications. These two technologies operate outside of the small signal regime wherein AC voltage distortion can be minimized.
With reference to FIG. 5, German Patent No. DE4401955 to Norbert discloses a circuit 500 for using polarized capacitors in transient AC applications. As taught by Norbert, circuit 500 is designed to be primarily a phase shifter for starting single-phase asynchronous motors. The circuit 500 is composed of AC source 501, anti-series pair 502, resistor 503, diode 504, inductive load 505 and switch 506. Diode 504 and resistor 503 are permanently connected to the AC voltage source 501 or alternately to a different negative voltage source. After a latency period with switch 506 open, the diode/resistor combination will gradually forwardly bias the capacitor pair. The Norbert circuit preconditions the capacitor for proper starting of the AC load, and increases the expected life over that of the Ghosh circuit when an adequate latency period is available prior to motor starting. Norbert allows the use of small diode ampacities relative to Ghosh. Norbert also proposes a high impedance connection to the anti-series capacitor center node, in an economic single can configuration. Only external diode, resistor and AC source connections are required to render the circuit ready for use.
Unfortunately, The Norbert circuit requires substantial time for capacitor biasing. The capacitors are charged to just under the magnitude of the AC voltage (peak to zero). For this reason, the Norbert circuit is incompatible with the use of low working voltage polarized capacitors in high AC system voltage applications. In addition, the circuit is unsuitable for use with other polarized charge carrier devices such as electrochemical batteries. Moreover, the Norbert circuit is unsuitable for continuous use in that the reforming charge tends to deteriorate over time if the single-phase motor or other load remains connected following start. The circuit will then behave identically to the conventional, uncharged anti-series configuration. The Norbert circuit will thus exhibit the disadvantage of clipping the AC waveform signal due to exceeding the small AC signal requirement in the steady state case.
Accordingly, a need remains for an improved method and circuit for using polarized charge storage devices such as polarized capacitors in AC applications including steady-state AC applications.